1. Field of the Invention
This invention relates to chemical injection systems for oil and gas wells. More particularly, it relates to a positive-displacement volumetric device for use in systems for injecting liquid-phase chemical treatment agents into undersea wells.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A variety of chemical agents are injected into hydrocarbon wells for the control of corrosion, hydrates, asphaltenes, paraffins, scale and the like. These chemical agents are typically in the liquid phase and are pumped into the well at a selected rate using a chemical injection system. For undersea wells, the chemical supply and pump may be located on a production platform and are most commonly connected to the wellhead via an umbilical line. If metering of the chemical agent is performed only at the surface, any leak in the umbilical or its connectors will give an erroneous indication of the quantity of chemical agent being injected into the well. Moreover, each subsea well may require its own injection system on the platform and connecting umbilical line.
Certain metering systems of the prior art employ a variable orifice—an adjustable orifice that allows remote control of flow at each well. Other metering systems of the prior art rely on pressure-compensated flow control—an adjustable pressure regulator and a fixed orifice can maintain a constant flow at each well.
Metering flow over a large range is often necessary over the life of the well. Orifice metering is limited in range and subject to filming, clogging and differing fluid properties.
Particulate contamination in long chemical injection lines is unavoidable and can clog the small orifices needed for metering and control. Filters on the lines are an added complication affecting system reliability, increasing capital costs and requiring periodic service (which increases operating costs).
U.S. Pat. No. 6,973,936 to Richard R. Watson (the disclosure of which is hereby incorporated by reference in its entirety) discloses a fluid injection system that controls the distribution of fluid from a supply line to a selected well at an adjustable rate. A free piston divides a cylinder into first and second chambers. A multi-position valve comprises a first position for passing fluid from the supply line into the first chamber to displace fluid from the second chamber back through the valve to an injection point, and a second position for passing fluid from the supply line to the second chamber to displace fluid from the first chamber back through the valve to the injection point. A control system in communication with a position sensor times displacement of the free piston to selected positions, and selectively adjusts a variable valve opening to adjust flow rate, switch between the first and second positions, and periodically increase the valve opening for cleaning.
The system disclosed in U.S. Pat. No. 6,973,936 may be characterized as a “fail closed” system—i.e., if power or control signals to the multi-position valve are interrupted, the system will continue to inject fluid into the well only until the free piston reaches the limit of its current stroke, at which point the flow of fluid will cease.
The present invention provides a “fail-as-is” state for a chemical injection system of the type disclosed in U.S. Pat. No. 6,973,936 in the event of a loss of power or control signals to the reversing valve. In a chemical injection system according to the invention, interruption of power or control signals to the valve results in a fluid flow rate substantially equal to the most-recently selected value. In this way, chemical treatment of the well can continue in the interim between the onset of the fault and its discovery and repair.